1. Field of the Invention
The present invention relates to a method of manufacturing a fused fiber coupler, particularly a pump coupler for fiber-optic amplifiers, wherein at least two single-mode fibers made according to DIN VDE 0888, Part 2, August 1987, Sec. 6.2, lie side by side in the same plane and form an input-fiber pair and an output-fiber pair that are fused together, and elongated under heat. Such couplers are used particularly in optical communication systems.
2. Description of the Prior Art
In "ELECTRONICS LETTERS", 12th April 1990, Vol. 26, No. 8, pages 523 and 524, a fused fiber coupler is described in an article by J. D. Minelly and M. Suyama entitled "WAVELENGTH COMBINING FUSED-TAPER COUPLERS WITH LOW SENSITIVITY TO POLARISATION FOR USE WITH 1480 nm-PUMPED ERBIUM-DOPED FIBRE AMPLIFIERS". It is made from a special fiber with a cutoff wavelength of 1450 nm and a numerical aperture of 0.2. This fused fiber coupler exhibits low sensitivity to polarization as is desired in fiber amplifiers, for example, but its fibers introduce increased attenuation at the splice to commercially available single-mode fibers as are employed for fiber-optic transmission links, because their mode-field diameters differ from those of the single-mode fibers. In addition, the special fiber is not available commercially, difficult to procure and, therefore, expensive,
In another article published in "ELECTRONICS LETTERS" 15th March 1990, Vol. 26, No. 6, pages 382 to 384, I. J. Wilkinson and C. J. Rowe report on fused fiber couplers in which polarization sensitivity was reduced by twisting the fiber pairs after tapering. Nothing is said about the type of fiber used, nor whether couplers with this property can also be fabricated for the wavelengths 1480/1550 nm.
Specialized dealers are offering wavelength-selective fused fiber couplers for different wavelengths and different channel spacings which are fabricated from commercially available single-mode fibers with a core diameter of 9 .mu.m or 10 .mu.m and a cladding diameter of 125 .mu.m, which conform to DIN VDE 0888, Part 2, August 1987, Sec. 6.2, for example. At particular wavelengths and particular channel spacings, however, the characteristics of these couplers are dependent on the direction of polarization of the light.
1480/1550 nm fused fiber couplers made from normal commercially available single-mode fibers are especially suited as pump couplers for fiber amplifiers. However, such couplers are highly polarization-sensitive. A negative effect of this sensitivity is that it causes gain variations.
The entire contents of all of the publications identified herein are incorporated herein by reference.